Wetter winters increase flood risk

It's raining harder and longer in the winter

		By Alex Kirby BBC News Online environment correspondent

UK scientists say weather records show prolonged spells of heavy winter rain are becoming more frequent.

They say the chance of several days' downpour has more than doubled since 1960.

They believe the change is contributing to a heightened flood risk; it matches a trend towards less rain in the summer.

The scientists believe both natural and human-induced climate change may be responsible.

Worst on record

The findings are the work of Dr Tim Osborn, of the Climatic Research Unit, University of East Anglia (UEA), and Dr Mike Hulme, of the university's Tyndall Centre for Climate Change Research.

Dr Osborn is presenting the research at a conference, Flood Risk in a Changing Climate, being held by the Royal Society, the UK's national academy of sciences, on 21 and 22 November.

The authors analysed rainfall data for the UK from 1961 to 2000, and found an increase during that time in the number of days of heavy winter rainfall, and a decrease in summer.

Dr Osborn said: "The floods [in the UK] in autumn 2000 were the worst on record in some places, and our analysis shows that this corresponds to a very high number of heavy rainfall days, particularly three or more days in a row.

Protracted downpours

"The number of days of heavy rainfall in winter is steadily rising, and increasingly these are coming in stretches of three days or more. This is very significant in terms of flood risk.

The 2000 floods were devastating

"The changes we are seeing in the pattern of rainfall in this country are likely to have been influenced by climate change, though natural climate variations probably also play a role."

Dr Osborn told BBC News Online: "We looked at three variables - the number of wet days, the intensity of the rain, and the number of consecutive days of rain.

"We've found only a very small change in the number of wet winter days. But the intensity of the rain and the spells of consecutive days it goes on are increasing.

"We're 50-60% likelier than we were in 1960 to get five or more days in a row of heavy winter rain.

Modelling discrepancy

"The summer rain intensity has remained about the same, though the average rainfall is decreasing.

Long-term evidence of more flooding is lacking

"Climate models show an increase in winter intensity, though the observed changes are larger than they suggest. So we suspect there's a natural factor too.

"Circulation changes, for instance to the North Atlantic Oscillation, and higher temperatures are leading to more evaporation and increased precipitation.

"The most obvious conclusion from our findings is that we've vindicated the climate models, and yet there's something else at work as well, perhaps a long-term natural trend."

% increase in very wet days per winter between 1961-1971 and 1991-2001

Stornoway 174%

Whitby 200%

Coltishall 100%

Hastings 35%

Swansea 25%

Hillsborough -15%

Slapton 8%

Another speaker at the conference, Dr Alice Robson of the UK's Centre for Ecology and Hydrology, is researching flood frequency.

She says she has found an increase in floods in the UK over the last 40 years, but no evidence of a trend over the longer term.

Dr Robson said: "The recent flood events do not look outstanding when compared with flood series that date back to early last century."